Conventionally, a thermo-sensing label is attached to main circuit connection of transformer station and electric power-applied section of high-voltage cable such that its color changes to indicate that a state of exceeding a predetermined temperature continues for a certain period for the purpose of constantly supplying electric power, thereby allowing the maintenance worker to visually monitor it.
FIG. 1(a) shows the schematic structure of a power transmission tower 2. It is composed of the power transmission tower 2 that supports transmission lines 1A, 1B and 1C (each having two transmission lines), transmission-line supporting portions 3 provided between the power transmission tower 2 and the transmission lines 1A, 1B and 1C, and an optical ground wire (OPGW) 4 provided on the top of the power transmission tower 2.
FIG. 1(b) shows partially the supporting portion 3. It is composed of an insulator 30 that insulates the power transmission line 1A from the power transmission tower 2, a cable clamp 31 that is supported through the insulator 30 by the power transmission tower 2 while holding the transmission line 1A, and a thermo-sensing label 11 that is attached to the surface of the cable clamp 31. The insulator 30 is fixed to an insulator fixing portion 31A of the cable clamp 31 and to a fixing portion of the power transmission tower 2. The thermo-sensing label 11 is also attached to the cable clamps 31 of the other transmission lines 1B and 1C.
FIG. 1(c) shows the details of thermo-sensing label 11. It is composed of a temperature indicating section 11A that is of a thermo-sensitive material whose color changes according to temperature, and temperature index sections 11B (red [higher than 65° C.]) and 11C (yellow [higher than 50° C.]) that indicate the relation of color indicated by the temperature indicating section 11A and the temperature. The temperature indicating section 11A has such irreversibility that the state of having the color once changed is retained. For example, when the temperature of cable clamp 31 is kept higher than 65° C. for a certain time due to electrical overload, its color changes to red and after that the color changing state is retained. Therefore, when it is found in the maintenance check that the thermo-sensing label 11 has the color changed, it can be visually and easily checked that the cable clamp 31 reached a high temperature. In case of transmission line, when loosing of a bolt in the cable clamp 31 for fixing the transmission line is generated by its vibration due to wind, the fixing portion may have the resistance value changed and generate heat. If power transmission is continuously conducted in such a state, the transmission line may be broken by heat.
Japanese patent application laid-open No. 5-66714 discloses a thermo-sensing label that has a temperature indicating section of a thermo-sensitive material with reversibility. In the thermo-sensing label, the transparency of the thermo-sensitive material varies between transparent and white opaque depending on temperature, and the thermo-sensitive material has reversibility in transparency. Therefore, the thermo-sensing label can be used repeatedly.
However, in the conventional temperature monitoring device using the thermo-sensing label, there is a problem that even when a periodical temperature change not reaching the extraordinary temperature occurs at the electric power-applied section due to an increase in amount of current carried, it cannot be detected since the device is designed to detect that its monitored object reaches an extraordinary temperature.
Also, in the conventional temperature monitoring system, it is necessary to visually check the color change of temperature indicating section. Therefore, in such a place that cannot be easily accessed by the maintenance worker for safety reasons, the object to be monitored will be subject to limitations. When remote monitoring by a camera etc. is employed instead of visually checking, it will be costly.
It is an object of the invention to provide a temperature monitoring device that can easily obtain temperature measurement results according to a situation of monitored object and that can safely at low cost monitor a temperature without having any monitored object limited during a long term.